Method of making a permanent magnet



May 21, 1963 H. T. MINDEN Erm METHOD oF MAKING A PERMANENT MAGNET FiledJuly 24, 1958 lNvENToRs F0/YARD BEREA/EAI/M B/ENRY AIM/06W ATTORNEYJMETHOD F MAKING A PERMANENT MAGNET Henry T. Minden, Roslyn Heights, andLeonard Berenbaum, Flushing, N.Y., assignors, by m'esne assignments,

to Sylvania Electric Products Inc., Wilmington, Del., a

corporation of Delaware Filed July 24, 1958, Ser. No. 750,725 2 Claims.(CL 29155.6)

'Ihis invention relates to improved permanent magnets. 'Ihere arevarious applications which require the use of v a permanent magnethaving a very high coercive force.

(The coercive force of a magnet is a measure of its ability to retainits magnetic strength under the inuence of an opposing magnetic field.)For example, in certain traveling wave tube applications, a plurality ofmagnets having a general toroidal or washer shape are stacked in suchmanner that the resultant magnetic field is axial and adjacent faces ofadjacent magnets have opposite magnetic poles. In order to produce thedesired field strength, the magnets so shaped and so arranged mustpossess a high coercive force.

Conventional materials from which permanent magnets are made, such asAluico V, lack the high degree of coercive force desired in theapplication described. Permanent magnets made of bismanol, however, dohave the required high degree of coercive force, but corrode rapidly andsuffer a permanent reduction in magnetic strength when exposed to ahumid atmosphere not uncommonly encountered. Eventually the corrosionprogresses to a point where cracking and pulverization of the magnetoccurs, rendering it useless as a magnet.

Accordingly it is an object of our invention to produce an improvedpermanent magnet having a very high degree of coercive force so that itwill retain its magnetic strength under the influence of demagnetizingfields.

Another object of this invention is to provide a method for makingmagnets having-a higher coercive force than conventional magneticmaterials arid which will not be subject to corrosion, deterioration orcracking.

Still another object is to produce apparatus for making improvedpermanent magnets of the type herein dcscribed. In accordance with theprinciples of our invention, a mixture of manganese bismuthide particlesand a binder is placed in a non-magnetic container. This container isthen placed in an apparatus where the binder 1s tirst subjected to asutiicient amount of heat to melt the binder material. While the binderis in a liquid state, pressure is applied to the mixture andsimultaneously a magnetic iield is applied to magnetically orient theparticles of manganese bismuthide therein in the direction of themagnetic lines of force. The mixture is then cooled to solidify thebinder while maintaining the pressure and the magnetic field. Thecontainer housing the magnet thus produced is then hermetically sealedto protect the magnetic substance from the atmosphere. A magnet is thusproduced which has a very high coercive force and magnetic strength andwhich is not deleteriously affected by atmospheric conditions.

Our invention will now be described in detail with reference to theaccompanying drawing which shows partly in cross section one embodimentof an apparatus employed to carry out the method of making the improvedmagnets.

Referring now to the drawing, there is shown a brass or othernon-magnetic metallic container toroidal in shape, which is lined withthin aluminum foil 11. A mixture 12 of manganese bismuthide and bismuth(bismanol) is then placed in the container and a at cover 14 is placedover the container. The loaded container is United States Patent Oice3,090,107 Patented May 2l, 1963 next placed on the top annular surface15 of a lower die member 16 which has a central portion 18 extendinginto the center region of the toroidal container 10. This portionsupports the inner sides of the container 10, when the mixture withinthe container is subjected to pressure as will later appear.

An upper die member 20 is next placed over the container cover 14 inalignment with the lower die member 16. The upper die member is providedwith outer and inner annular grooves 24 and 26 respectively forreceiving any molten overliow squeezed from the when heat and pressureare applied thereto in accordance with the method to be described. Byproviding annular grooves to collect the overow, it is easier to break'the container freefrom the upper die member when the container 10process is complete. The central bore of the upper die member terminatesat one end in a conical shape 28 td facilitate more even distribution oflines through the upper die member 20. Lower and upper co1- lar members30 and 32 are disposed about the lower and upper die members 16 and 20respectively, and support the external surface of container 10 whenpressure is applied thereto. v

Surrounding the upper and lower die members in the region of thecontainer 10 is an electric heater 34`having wires 35 for heating themixture in the container 10. A thermocouple 36 is located in the lowerdie member 16, which, in conjunction with an automatic controller (notshown) maintains the mixture at the proper temperature. Cooling coils 38which carry circulating cold water are provided about the lower portionof the lower die member for cooling the mixture at the proper time. Thecooling coils 38 are located some distance below the level of thecontainer 10, so that the mixture can be cooled from the bottom of thecontainer 10 rather than from its sides. Such directional cooling andthe use of the aluminum foil 1,1 to eliminate sticking of the mixture tothe inner wall of the container, prevent cracking of the magnet duringthe cooling process.

The upper and lower die member assembly just described is supported onthe top face 39 of a lower pole piece 40, which is part of a largeelectromagnet comprising a lower pole piece 40, an upper pole piece 42,a yoke portion 44, and current carrying coils 46 disposed about both thcupper and lower pole pieces. The upper yoke member 42 is slidably heldby the yoke 44 to allow vertical movement thereof so that the face ofthe upper pole piece 42 can be brought into direct contact with the topof the upper die member 20. Provision is also made to apply pressurevertically from a press, not shown, through a bar 48, the upper polepiece 42, upper die member 20 and the container cover 14, to compressthe contents 12 in the container -10 thereby densifying the same.

The process is conducted in the following manner. The bismanol powder12, which is composed of particles of manganese bismuthide and bismuthis loaded into the non-magnetic toroidal container 10. This is doneunder the protection of nitrogen to prevent reaction with theatmosphere. The cover 14 is then placed on the container 10. The loadedcontainer is placed on the top surface 15 of the lower die member 16which has disposed about it the lower collar 30, heater 34 and coolingcoils 38. The upper die member 20 is then placed over the containercover 14, the upper collar 32 is placed in position and the upper polepiece 42 is lowered into contact with the top of the upper die member20.

The heater 34 is then turned on and the temperature is automaticallyregulated by a controller in cooperation with the thermocouple 36. Themixture is heated to 300 C., a temperature above which bismuth melts ofmagneticfFlf-Li 3 (271 C.) butbelow the Curie temperature of manganesebismuthide (360 C.), which is the temperature above which the lattermaterial will lose its permanent magnetism.

While maintaining a temperature of 300 C., the manganese bismuthideparticles become dispersed in the liquid bismuth medium and aremagnetically oriented and then permanently retained in this position byfreezing the liquid bismuth, as will now appear.

When the melting point of bismuth is reached, the magnetizing coils 46are energized to create a strong magnetic ield to thus magnetically'orient the manganese bismuthide particles with respect to the directionof the applied magnetic iield so that upon removal of the tield, astrong permanent magnet will have been produced in the direction of theaxis of the toroid. While the magneticiield is applied, the mixture isheld at a temperature of ap- .proximately 300 C. In order to insure theproper orientation of all the manganese bismuthide particles, the mag-'petic field should be maintained for a sutcient time gvhile the bismuthis molten. This time varies fromsevmi minutes for very thin magnets toseveral hours for very thick ones.

While the bismuth is molten, a pressure is applied through the. bar 48to increase the density of the contents of the containerlO. Themagnitude of the pressure is a variable depending upon the thickness ofthe magnet. After sulhcient healing time has elapsed, the heater 34 isturned off and the container is cooled by passingvcold water through thecooling coils 38. Cool air may also be blown over the apparatus to speedup the cooling process. To achieve maximum magnetic strength, it isimportant to maintain the magnetic iield while the molten bismuth isabove its melting point to insure that the man ganese bismuthideparticles are frozen in their properly oriented positions.

After the magnet is cooled, it is removed from the apparatus and sealedto protect the manganese bismuthide from the deleterious eteets of theatmosphere. The sealing may be carried out by low temperature solderingor 4 react with the latter. Plastics and other compositions can also beused.

A modification of our process can be carried out by forming the magnetin the desired shape without the use of a container and subsequentlysealing the magnet off from the deleterious effects of the atmosphere bythe use of a protective coating.

What is claimed is:

1. The method of making a permanent magnet comprising, charging atoroidal container of non-magnetic material lined with aluminum foil andhaving a toroidal cover with a powder containing particles of manganesebismuthide and bismuth, heating said powder to a temperature suicient toform a dispersion of manganese bisany other suitable method so long asthe method does j.: not result in4 melting of the bismuth or inotherwise decreasing the strength of the magnet.

By this method a permanent magnet is produced, which has a high magneticstrength and also a much higher coercive force than conventionalmagnets. While our process has been described using bismuth as a binderto maintain the manganese bismuthide particles magnetically oriented intheir proper positions, it should be understood than other substancescan also be used. For example, other non-magnetic metals having amelting point somewhat below 360 C., the Girie temperature of manganesebismuthide, would be suitable if they do not muthide particles in moltenbismuth but below the Curie -temperature of said manganese bismuthide,applying a magnetic field to said dispersion to magnetically orient said.particles of manganese bismuthide with respect to said applied magnetic`ield, applying pressure between said toroidal cover and the bottom ofsaid container to densify said solution, and direetionally'cooling saidsolution to solidify said bismuth while maintaining said manganesebismuthide particles so oriented, and hermetically sealing the cover onsaid container of non-magnetic material lined with aluminum foil, toprotect the magnet so produced from the atmosphere. l Y

2. The method of making a permanent magnet cornprising dispersingparticles of a ferromagnetic material in a liquid binder held in anon-magnetic container having a cover, said binder being held at atemperature suiicient to maintain said binder in a liquid state butbelow the Oirie temperature of saidA ferromagnetic material, subjectingsaid ferromagnetic material to the inuence of a magnetic field tomagnetically orient said ferromagnetic particles with respect to thedirection of said applied magnetic field, densifying said dispersion,directionally cooling and freezing said binder while maintaining saidferromagnetic particles so oriented to permanently tix said particles insaid oriented direction, and sealing the cover on said container ofnon-magnetic material to protect lits contents from the atmosphere.

References Cited in the tile of this patent UNITED STATES PATENTS2,576,679 Guilland Nov. 27, 1951 2,589,766 Bradley Mar. 18, 1952,2,698,9-17 Van Urk et al. Ian. 4, 1955 2,751,525 Hekelaar .Tune 19, 19562,825,670 Adams et al Mar. 4, 1958 2,849,312 Peterman Ang. 26, 19582,862,287 Koch et al. Dec. 2, 1958 2,865,085 Cornish Dec. 23, 19582,892,M8 Weber et al. June 30, 1959

2. THE METHOD OF MAKING A PERMANENT MAGNET COMPRISING DISPERSINGPARTICLES OF A FERROMAGNETIC MATERIAL IN A LIQUID BINDER HOLD IN ANON-MAGNETIC CONTAINER HAVING A COVER, SAID BINDER BEING HELD AT ATEMPERATURE SUFFICIENT TO MAINTAIN SAID BINDER IN A LIQUID STATE BUTBELOW THE CURIE TEMPERATURE OF SAID FERROMAGNETIC MATERIAL, SUBJECTINGSAID FERROMAGNETIC MATERIAL TO THE INFLUENCE OF A MAGNETIC FIELD TOMAGNETICALLY ORIENT SAID FERROMAGNETIC PARTICLES WITH RESPECT TO THEDIRECTIN OF SAID APPLIED MAGNETIC FIELD, DENSIFYING SAID DISPERSION,DIRECTIONALLY COOLING AND FREEZING SAID BINDER WHILE MAINTAINING SAIDFERROMAGNETIC PARTICLES SO ORIENTED TO PERMANENTLY FIX SAID